Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
ADC analog to digital converter
ASIC application specific integrated circuit
dB decibel
dBFS decibel full scale
DSP digital signal processing
ECM electret condenser microphone
EMC electromagnetic compatibility
Gnd ground
L/R left/right
MEMS micro electro-mechanical system
PDM pulse density modulation
SNR signal to noise ratio
SPL sound pressure levels
Vdd supply voltage
Audio-visual camcording is one of the most common uses for a mobile device with multimedia capabilities. Advances have been made with respect to the visual capturing components. Customer expectations have in turn risen in regards to the quality of video recordings, however the audio components including both capture and playback have lagged behind the visual side. This may be especially problematic when capturing video events that have high sound pressure levels (SPL). The internal microphone of the mobile device can saturate easily when trying to record high SPLs, which are also frequency dependent. The recorded audio signal can result in heavy distortion and compression which influences the overall audio quality.
Audio distortion may also occur in windy environments. The movement of air across a microphone element of a mobile device may generate unwanted noise. This unwanted noise may reduce the intelligibility of the audio and force a user of the mobile device to terminate a phone call prematurely.
Currently some mobile devices with multimedia capabilities fail to provide sufficient audio capture in a high SPL environment. The quality of audio capture often depends on the microphone arrangement and microphone design. One possible arrangement involves implementing multiple microphones on a single base as is shown in FIG. 1, which is a reproduction of FIG. 3A of US Patent Application Publication US 2007/0047746 A1. The microphone system shown in FIG. 1 has a base 130, and a plurality of substantially independently movable membranes 120 secured to the base 130. Each of the plurality of membranes 120 forms a variable capacitance with the base 130. Thus, each membrane 120 effectively forms a generally independent, separate microphone with the base 130.
Another possible arrangement involves a direct digital microphone that is constructed of a plurality of first membranes 220 each formed by a micro-machined mesh supported by a substrate 270. This is shown in FIG. 2, which is a reproduction of FIG. 3 of US Patent Application. Publication US 2003/0210799 A1. A second membrane 210 and a plurality of first membranes 220 are located in two different positions. The plurality of first membranes 220 is comprised of individual first membranes 260. The second membrane 210 is supported by a substrate 270 and positioned above the plurality of first membranes 220 to form a chamber 230 between the plurality of first membranes 220 and the second membrane 210. A pressure sensor 240 is responsive to pressure in the chamber 230. Drive electronics 250 are responsive to the pressure sensor 240 and control the positions of the plurality of first membranes 220. Polling electronics 250 are responsive to the positions of the plurality of first membranes 220 and produce a digital output signal.